This invention relates to winches, in particular winches used in a marine environment. It provides a means for maintaining a substantially constant tension on the winch rope despite the fact that the load and the winch are moving relative to each other. It further provides a means for safely raising a load from, or lowering a load onto, a vessel moving on the sea surface relative to the crane which is raising or lowering the load.
There are a number of applications in which it is necessary or desirable to maintain a constant tension on a winch rope, despite the fact that the winch and the load are moving relative to each other. Examples are preventing shock loads on a marine crane when lifting is initiated, maintaining a constant pressure on a drill stem being operated from a floating rig and maintaining a steady tension on a marine anchor.
A number of servo-controlled systems intended to automatically operate the winch so as to follow the movement of the load and thereby maintain a constant tension are described in the prior art. Examples can be found in U.S. Pat. Nos. 2,249,947 and 3,753,552. However, all such mechanical or electro-mechanical control systems have an inherent lag between input of a command to alter the operation of the winch motor and the alteration actually occurring. This lag means that the tension cannot be maintained constant, particularly at a point where the relative direction of motion changes. The conventional response to this problem is to design the control system so as to have as small lag as possible under the circumstances. However, in the types of application described above, the winch loads are typically large, and to control such loads the winch motor and controller must be of a design that typically has a relatively long lag time.
Further problems are experienced in situations requiring the removal of loads from or the deposition of loads onto the surface of a vessel using a crane which is relatively stationary, particularly in heavy seas. However, it may be impossible in some circumstances to delay such operations until the sea is relatively calm. Examples of such situations are the supply of offshore drilling and production platforms, particularly in storm-prone areas such as the North Sea, and military operations.
The problems arise mainly because the vessel is rising and falling with the waves, and this can represent considerable vertical movement and acceleration of the vessel's surface relative to the crane which is to hoist the load from, or lower the load onto that surface. In the case of hoisting, initiation of the lift at an inappropriate point of the vessel's movement can result in severe shock loads on the crane, which may result in failure of components, damage to the crane or, in extreme cases, toppling of the crane from its mounting. As well as damage to the crane, the crane operator may be injured or even killed. Further, the vertical acceleration of the vessel caused by wave movement may be such that the surface of the vessel comes into secondary contact with the load after lifting has commenced. This can cause severe damage to the load, the vessel, or both, and can also cause serious injury to persons on the vessel.
In the case of lowering the load, if the load contacts the surface at a point where the vessel is accelerating upwards, the load or the vessel may be damaged, and there is considerable danger to the deckhands who are preparing to receive the load.
Further difficulties are caused because ocean waves are not a perfect sine wave, but vary in height in a way that is not accurately predictable. In heavy seas it also often happens that one wave may be superimposed slightly out of phase on another, resulting in a false crest. If lifting or lowering occurs on such a false crest, secondary contact is almost certain to occur.
In order to minimize these dangers, the load should be hoisted from or lowered onto the surface at the moment when the vessel is near the crest of a major wave. In heavy seas it is almost impossible for the crane operator to make an unassisted determination of this optimum time. He is generally located in a cab on the crane, high above the surface of the vessel which is almost vertically below him. Further, he must be constantly operating the crane controls while the load is being prepared for hoisting so as to keep the hoist rope vertically positioned above the load to prevent the development of a pendulum movement of the load when it is hoisted. In addition, visibility is likely to be poor in severe sea state conditions.
As a result of the problems outlined above, there have been numerous accidents which have occurred during the transfer of suplies to offshore platforms, particularly in the North Sea, resulting in extensive property damage, personal injuries and loss of life. In order to reduce the number of these accidents, the relevant authorities have enacted regulations, governing such things as the maximum sea state that supply operations can be carried out in, and specifying the factor by which a crane must be derated according to sea state. The result of these regulations is that there are significant limitations on what can be lifted onto an offshore platform and the conditions in which the lifting operation can take place.
Various devices for determining the correct point for initiating hoisting have been described, for example in U.S. Pat. Nos. 4,098,082, 4,304,337 and 4,324,385. None of this prior art, however, addresses the problem of distinguishing between true and false crests, and between sizes of waves in the same train. Further, all the prior art referred to above is directed to the problem of unloading a vessel by a stationary crane, and does not address the different problems which arise when the reverse maneuver is to be carried out.